METHODS OF RESEARCH 21 



messenger is sent down to release the larger 

 cube, which falls and covers the smaller one, 

 thus ending the exposure. The apparatus is 

 hauled up and the cubes taken into the dark 

 room for the development and fixing of the 

 plates. 



Current Measurements. — For measuring the 

 velocity and direction of currents in the sea 

 many devices have been employed. Informa- 

 tion regarding surface currents may be 

 obtained !rom the drift of floating objects, 

 such as drift-bottles, wreckage, icebergs, 

 vessels frozen in the ice of polar regions, and 

 movements of water-masses at the surface and 

 under it may often be traced by studying 

 their physical and chemical properties (tem- 

 perature, salinity, dissolved gases). For the 

 direct measurement of undercurrents it is 

 necessary to use a current meter of somewhat 

 complicated construction. The latest form is 

 that designed by V. W. Ekman. 



Hydrometers. — The direct determination of 

 density may be made by means of the hydro- 

 meter, a glass cylinder which floats in water. 

 Densities so found are re-calculated by means 

 of tables to a standard temperature. Owing 

 to the uniform composition of sea-salts a 

 definite density at a definite temperature 

 corresponds rigidly to a definite salinity. 

 Hence, by referring to tables the salinity of 



